1. Field of the Invention
This invention relates to a switching power source in which the error of an output voltage is fed back to the primary side through an insolation element, and more particularly, to a switching power source in which a power outage detection signal is produced according to a signal indicating the error of the output voltage.
2. Description of the Related Art
In general, a power outage detecting circuit in an industrial control device, comprises: a photo-coupler for detecting both waves of a commercial power source; and a timer (such as a mono-stable multi-vibrator) which is triggered by the output of the photo-coupler and can be triggered again.
That is, the timer is triggered by every wave (of both waves) of the commercial power source. Hence, the timer is reset before the time counting period is ended so that the time counting operation is performed again from the beginning. As a result, normally, the timer output is maintained at the level indicating a time counting state (a level indicating a non-power-outage state).
On the other hand, when power outage occurs, the timer is not triggered. Therefore, when the time counting period passes which is set, for instance, to two cycles of the commercial power source, the time counting operation is ended, and the output level is inverted; that is, the output level is changed into the level which indicates the occurrence of power outage.
The above-described circuit is able to detect the power outage before the output voltage of the switching power source is decreased; that is, it is able to detect the occurrence of power outage considerably quickly. However, the circuit arrangement is complex because it needs an insolation element, namely, a photo-coupler, and a timer which can be triggered again.
In order to prevent the circuit arrangement from becoming complex, a switching power source used in a device for consumers is so designed that the output voltage of the secondary side is utilized for detection of the occurrence of power outage. FIG. 4 shows a conventional detecting circuit which utilizes the output voltage of the secondary side to detect the occurrence of power outage (the primary side circuit, and an error detecting circuit are not shown).
As shown in FIG. 4, the base of a transistor Q11, which is provided for the detection of the occurrence of power outage, is connected through a Zener diode ZD (hereinafter referred to merely as "a diode ZD", when applicable) to a negative output 31. The Zener voltage of the diode ZD is so determined that the sum of the base-emitter voltage of the transistor Q11 and the Zener voltage of the diode ZD is lower about several volts than the difference voltage between the output 32 (which is 5 V) and the negative output 31. Hence, during the non-power-outage period, a base current flows in the transistor Q11, and a power outage detection signal 33 is at "H" level.
On the other hand, upon occurrence of power outage, the voltages of the negative output 31 and the output 32 are decreased. When the difference voltage between the output 32 and the negative output 31 becomes smaller than the sum of the base-emitter voltage of the transistor Q11 and the Zener voltage of the diode ZD, the transistor Q11 is turned off, so that the level of the power outage detection signal 33 is changed to "L" level indicating the occurrence of power outage.
In the above-described conventional detecting circuit, a smoothing capacitor C11 for the negative output 31 is small in capacitance, and therefore when power outage occurs, the voltage of the negative output 31 is quickly decreased. Accordingly, the current flowing in the diode ZD becomes zero quickly, and the transistor Q11 is turned off.
FIG. 5 is a graphical representation indicating changes in voltage of the output 32 with changes in level of the power outage detection signal 33 in the above-described conventional detecting circuit.
That is, the capacitor C11 being small in capacitance, the voltage of the negative output 31 is quickly decreased, and therefore at the time instant T12 when the voltage of the output 32 is not so decreased, the power outage detection signal 33 is set to "L" level. However, at the time instant T12, the voltage of the output 32 starts to decrease. Therefore, after the time instant T12, the period of time is extremely short for which the voltage of the output 32 maintains the power source voltage of the microcomputer at the specified value. Hence, the time margin for the microcomputer to treat the power outage is short.
In order to increase the time margin, the rate of decrease of the voltage of the output 32 should be decreased. For this purpose, smoothing capacitors C12 and C13 for the output 32 should be increased in capacitance. However, if the smoothing capacitors C12 and C13 are increased in capacitance, they are increased in price, and the resultant detecting circuit is increased in size as much.
On the other hand, there is a method in which the capacitor C11 is decreased in capacitance thereby to shorten a period of time t11. However, since the capacitor C11 is used for smoothing the voltage of the negative output 31, and therefore its capacitance is limited in minimum value. This makes it impossible to shorten the period of time t11 so much.